The night before: roll 80g of white fondant into a tapering cone horn shape around a wooden skewer. Shape two fondant ears with pink centers. Dry all pieces overnight on parchment. Paint the dried horn gold with luster dust mixed into vanilla extract.
On baking day: preheat oven to 175°C (350°F). Grease and line two 9-inch cake pans.
Beat butter and sugar on medium-high for 3 to 4 minutes until very pale and fluffy. Add eggs one at a time on medium-low, then the egg white and vanilla.
Whisk flour, baking powder, and salt together. Add to the batter in three additions, alternating with milk in two, mixing on lowest speed after each. Stop when just combined.
Divide batter equally into 4 bowls (approximately 180g each). Tint each bowl a different gel color. Drop spoonfuls of each color alternately into both prepared pans. Bake 28 to 32 minutes until a toothpick comes out clean. Cool 15 minutes in pans, then fully on a wire rack.
Make white buttercream: beat butter 2 minutes, sift in powdered sugar in three stages, add vanilla, salt, and cream. Beat until silky.
Make pastel mane buttercream the same way, divide into 4 to 5 bowls, tint each a different pastel color. Spoon each into a piping bag fitted with a large star tip.
Stack cooled layers with white buttercream between them. Apply a crumb coat, refrigerate 15 minutes, then apply the final smooth white coat.
Insert the horn and ears. Pipe the pastel mane with alternating rosettes and drop stars from the horn base cascading down one side. Scatter edible sprinkles through the mane. Draw closed eyes with lashes on the front using the edible ink marker.
